In preparation for adding programmable font support for EGA and VGA video cards, I (re)discovered that the IBM VGA ROM font that PCjs was using was an 8x14 font, not the “higher resolution” 8x16 font introduced with the VGA. This was by virtue of hard-coded ROM font offsets that all PCjs machines were passing to the Video component:
<rom id="romVGA" addr="0xc0000" size="0x6000" file="/devices/pcx86/video/ibm/vga/1986-10-27/ibm-vga.json" notify="videoVGA[0x378d,0x3f8d]"/>
After the Video ROM is loaded, the Video component is passed an array containing two addresses:
- : 0x378D, the offset of the 8x8 ROM font in the ROM
- : 0x3F8D, the offset of the 8x14 ROM font in the ROM
For example, if we want to look at the 8x8 data for the character
0 (ASCII 0x30), we can dump it as binary,
using the PCjs Debugger:
>> dby c000:378d+30*8 l8 &C000:390D 7C 01111100 &C000:390E C6 11000110 &C000:390F CE 11001110 &C000:3910 DE 11011110 &C000:3911 F6 11110110 &C000:3912 E6 11100110 &C000:3913 7C 01111100 &C000:3914 00 00000000
Ditto for the 8x14 data (note that 14 must be written as “14.” for the Debugger to interpret it as decimal instead of hex):
>> dby c000:3f8d+30*14. l14. &C000:422D 00 00000000 &C000:422E 00 00000000 &C000:422F 7C 01111100 &C000:4230 C6 11000110 &C000:4231 CE 11001110 &C000:4232 DE 11011110 &C000:4233 F6 11110110 &C000:4234 E6 11100110 &C000:4235 C6 11000110 &C000:4236 C6 11000110 &C000:4237 7C 01111100 &C000:4238 00 00000000 &C000:4239 00 00000000 &C000:423A 00 00000000
If you squint a little at all the 1s and 0s, you should be able to make out the shape of a zero with a diagonal slash through it.
By setting a write breakpoint on A000:0000 (
bw A000:0000) and triggering another video mode set (eg, with the DOS command
MODE CO80), we can easily determine that the source of the 8x16 font data begins at C000:4EBA, so the bits for character
0 are here:
>> dby c000:34eba+30*16. l16. &C000:51BA 00 00000000 &C000:51BB 00 00000000 &C000:51BC 38 00111000 &C000:51BD 6C 01101100 &C000:51BE C6 11000110 &C000:51BF C6 11000110 &C000:51C0 D6 11010110 &C000:51C1 D6 11010110 &C000:51C2 C6 11000110 &C000:51C3 C6 11000110 &C000:51C4 6C 01101100 &C000:51C5 38 00111000 &C000:51C6 00 00000000 &C000:51C7 00 00000000 &C000:51C8 00 00000000 &C000:51C9 00 00000000
Note that this zero differs from the other two, because it has a “dot” in the center rather than a slash. It seems strange that IBM would switch styles like that between the 8x14 and 8x16 fonts on the same card, but then again, IBM made lots of seemingly arbitrary and sometimes irritating decisions in those days, like swapping the positions of the Ctrl and Caps-Lock keys when they introduced their 101-key keyboards. But I digress.
Later in the VGA ROM’s initialization sequence, the font is “upgraded” to a 9-bit wide version, since VGA monitors were capable of displaying text screens with a horizontal resolution of 720 pixels, just like the original IBM monochrome monitors.
>> dby c000:5ecc l16. &C000:5ECC 00 00000000 &C000:5ECD 00 00000000 &C000:5ECE 3C 00111100 &C000:5ECF 66 01100110 &C000:5ED0 C3 11000011 &C000:5ED1 C3 11000011 &C000:5ED2 DB 11011011 &C000:5ED3 DB 11011011 &C000:5ED4 C3 11000011 &C000:5ED5 C3 11000011 &C000:5ED6 66 01100110 &C000:5ED7 3C 00111100 &C000:5ED8 00 00000000 &C000:5ED9 00 00000000 &C000:5EDA 00 00000000 &C000:5EDB 00 00000000
Unfortunately, drawing text with IBM’s 9x16 font doesn’t look that great on our default VGA canvas. This is because PCjs uses a canvas with an aspect ratio of 1.33, since that matches the aspect ratio of the VGA’s high-resolution graphics mode (640x480). VGA text modes, on the other hand, use a resolution of 720x400, which has an aspect ratio of 1.8.
So our choices are: 1) dynamically alter the aspect ratio of the canvas to match that of the current text or graphics mode, or 2) horizontally squeeze and vertically stretch 720x400 text so that it fits on our 640x480-shaped canvas.
I don’t like the idea of “reshaping” the canvas – after all, a physical VGA monitor doesn’t change its shape whenever the video mode changes – so I rely on the browser’s 2D APIs to automatically scale the text to the canvas. But by today’s standards, a 9x16 VGA font is rather “low-res”, so when it’s scaled, the results aren’t all that smooth.
Image Smoothing To The Rescue
It turns out there’s another choice: 3) set the imageSmoothingEnabled property to true for the 2D context of the display canvas. Problem solved.
I had visited this issue once before (see the PCjs blog post “The Sharpening”), and at that time, I had decided to set imageSmoothingEnabled to false for most machines, because smoothing tended to make PC graphics screens look a bit fuzzy and dull.
However, it’s become clear that smoothing does make sense for PC text modes. It may also help that all PCjs fonts are now rendered internally at twice their normal resolution (eg, a 9x16 VGA font is rendered internally at 18x32), to help them look less thin and anemic. This automatic font-doubling feature (controlled by the Video component’s fDoubleFont property) used to be selectively enabled based on another now-deprecated Video property (scale) and some overly-complicated virtual vs. physical screen size comparisons.
Now fDoubleFont is hard-coded to true, and imageSmoothingEnabled is re-enabled just for text modes.
Apr 5, 2018